1. Field of the Invention
The present invention relates to an optical disk apparatus and, more particularly to an optical disk apparatus wherein data can be optically recorded on, and reproduced from, the tracks of a memory disk while the disk and an optical head are moving relative to each other.
2. Description of the Related Art
Optical disk apparatuses have been in practical use, each typically having an optical head comprising a semiconductor laser and an objective lens. The laser emits a beam, and the objective lens focuses the laser beam onto a track of an optical disk which is spinning and located near the optical head, thereby to record data on the track or to reproduce data therefrom. The optical head undergoes focus control so that the objective lens focuses the laser beam appropriately. Also, the head undergoes tracking control so that the laser beam focused by the objective lens is applied onto a desired track.
The optical disk apparatus further has a linear motor and an objective drive mechanism, as is disclosed in U.S. Pat. No. 4,037,252. The linear motor moves the optical head in the radial direction of the optical disk, when the apparatus is set in a coarse access mode. The objective drive mechanism also moves the objective lens toward or away from the optical disk, when the apparatus is set in the fine access mode.
To access any target track of the optical disk, the apparatus is first set in coarse access mode. In the coarse access mode, the linear motor moves the optical head toward the target track. The head applies a laser beam to the optical disk. The beam is reflected from one of the tracks, and the position of this track is determined from the reflected laser beam. If the position thus determined greatly differs from the position of the target track, the apparatus is continuously set in the coarse access mode. Hence, the linear motor is driven again, moving the head toward the target track. If the position differs a little from that of the target track, the access mode is changed to the fine access mode. In this case, the objective drive mechanism moves the objective lens to focus the laser beam, thus achieving a fine access to the target track.
The optical disk apparatus further comprises a position detector. While the apparatus is set in the coarse access mode, this detector reads, or scans, the optical scale mounted on the optical head, thereby determining the position of the head and, thus, the distance the head has been moved.
The optical disk apparatus, described above, has two drawbacks. First, once the optical head is vibrated due to an external force and fails to track a target track after it has been moved by the linear motor or during the optical reproduction of the recorded data, it can no longer track any track. Second, the optical head can hardly accomplish a stable tracking, for the reason discussed in the following paragraph.
Since the objective lens is suspended from a fixed member by a wire suspension, it is vibrated in the radial direction of the optical disk when the head is stopped, due to the deceleration of the linear motor. If the laser emits a beam onto the disk while the objective lens is vibrating, the position of the track to which the head has been moved cannot be detected correctly. Hence, the laser is not turned on, or the objective drive mechanism does not move the objective lens, until a period of time (about 20 msec), which is slightly longer than the time the lens requires to stops vibrating, elapses after the optical head has been stopped. In other words, the tracking of the disk is started upon lapse of a predetermined time after the head has been stopped, regardless of the relative speed between the head and the disk. When this relative speed is higher than the normal value because the disk is eccentrically located with respect to the spindle of the disk apparatus, or because the lens is vibrating when the linear motor is stopped, a tracking error signal generated from the laser beam reflected from the disk will have a frequency exceeding a predetermined tracking-servo range. Consequently, the optical head cannot achieve a stable tracking.